A underwater connection between the Island would be more then unrealistic.
The depth of the water behind the dam would be more or less the same as the height of the dam itself. Noone would even try to build something inside that.
The dam itself allready offers 3 routes: on top of the dam, inside of the dam and the pier behind the dam. The underground area inside the dam can be expanded so that there are actually rooms instead of only corridors. Maybe it could even have more then one underground level.
If you wanna expand the map to use the area where bond starts in SP too, you could try to connect these two "rooms" with the underground area.
The only thing that cannot be avoided is the long open walkway on top of the dam.
But it might be possible to add some cover by placing a truck and maybe also other stuff on there.
If you wanna have the island reachable (good sniper spot), i'd say link it with a passage from the place where Bond starts in SP.
Don't feel restricted to much to the original map, since it had to be built with tough hardware limitations in mind.